1. Field of the Invention
The present invention relates to devices which simulate the failure of materials used in structures so that failure can be predicted. More particularly, the invention relates to devices using acoustic emission techniques for simulating failure.
2. Description of the Related Art
Structures such as bridges, buildings and aircraft are constantly subjected to high stresses and corrosion. Structures can fail because of high stress and corrosion. Structural failure occurs when cracks develop and grow in a structure due to stress. Prediction of structural failure is desirable since it can save lives and property. However, prediction of structural failure is not easy. One cannot always tell from looking at a structure whether a structure is prone to failure. Often, a structure cannot be viewed because it is hidden, such as the structure of a concrete building or an airplane.
Structural failure could be predicted by removing a portion of a structure and testing the portion removed. This kind of method is known as a destructive testing method. Such a method, however, is impractical since the removal of a sample from a structure would further weaken the structure.
Because of the impracticality of destructive testing methods, nondestructive testing methods have been developed. One widely used nondestructive testing method which is very reliable is the use of acoustic emission technology. Acoustic emission technology is based on the fact that when a crack in a structure propagates, it produces an acoustic emission. If these emissions can be detected, the damage to a structure can be measured. Thus, acoustic emission technology can be used to predict structural failure.
Acoustic emission techniques involve attaching high frequency ultrasonic transducers (20 Khz-500 Khz) to a structure under stress to listen for the high frequency sound waves released when a crack propagates in the structure. In order for acoustic emission monitoring to be effective, especially when continuously monitoring a structure, it is necessary to assure that the signal due to propagation of the crack can be distinguished from background noise which is always present during operation. Additionally, it is necessary to determine the attenuation of crack-like acoustic emission signals within the structure in order to determine what spacing to use for the transducers in order to assure that complete coverage of the structure is accomplished. Finally, it is necessary to determine if a proper failure model is present in the system software to predict that failure of the structure is imminent, so that it can be unloaded prior to catastrophic failure. These requirements can be easily accomplished by simulating actual acoustic emission signals from a crack while monitoring the structure for the simulated signals.
The main factor hindering the application of acoustic emission techniques to the testing of critical structures is the lack of a simulated signal source with the required bandwidth to simulate the growth of a crack in a critical structure. One method for simulating the required bandwidth involves placing a transducer similar to the transducer being used to detect the acoustic emission signals from crack growth on the structure and pulsing the transducer with a step voltage pulse. This causes the transducer to send out an artificial acoustic emission signal throughout the structure. The problem with this technique is that the frequency content of the signal sent out is a very narrow band centered around the resonant frequency of the sending transducer, unlike a wide band signal from a growing crack.
Another method for simulating the required bandwidth is the use of a pencil, where a small piece of pencil lead is broken when the pencil is pressed against the structure. This technique more closely simulates the signal from a growing crack but it is very awkward to use, and is impossible to use in many situations involving real structures.